For Cutting the ends out, I use a sharpened cold Chisel.
Taking my time and being careful, I can still cut one out in well under 10 minutes and you don't have any of the dust and crap you do if you use a grinder.

If the drum has had anything flammable in it just fill it with water which stops the drum moving around and supports it as well which makes for easier cutting.

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I don't give a damn about what might or could happen until a significant group of people can tell me it HAS happened to them.
Until then, it's just more endless gloom and doom Veg folk law.

Yes a sawzall is the tool of choice to cut the drum in half but getting the cut started is hard even with a new blade. I found that drilling three or four small pilot holes along the cutting line aids the blade to get through the steel. Once through it's a piece of cake. I use a drum de-header (can opener on steroids) for removing the lid (after I turned it up side down so I'm removing the bottom to keep the two bungs intact). For small holes I use pilot hole then a step drill bit.

I use a large engine intake valve for taking the tops out. the edge seems to take being tapped with a hammer and it leaves a nice finish like a can opener. Hold the stem out over the edge of the drum and just tap it around
John